The printing ink compositions of the present invention are particularly suited for use in lithography. Lithography is a printing process that relies on a chemical distinction between image and non-image areas of the printing plate rather than any physical relief differentiation. Such plates are referred to as planographic since the image and non-image areas are in the same plane. The plates are constructed so that with proper treatment the image areas are hydrophobic and oleophilic and thereby receptive to inks. The non-image areas are hydrophilic and are water receptive.
With the exception of driographic, i.e. waterless, plates, all types of lithographic plates require a continuous supply of an aqueous fountain solution to activate and maintain the distinction between ink-accepting image areas and ink-repelling non-image areas.
As regards lithography, the distinction between image and non-image areas is sufficient to cause the following effects:
1) aqueous fountain solution spreads on the non-image area; PA1 2) aqueous fountain solution in the image area is unable to form a continuous film, leaving the image free to accept ink. PA1 (a) a dispersion of about 10-25 wt. %, preferably 13-22 wt. %, based on the weight of the composition, of a pigment in about 40-90 wt. %, preferably 50-80 wt. %, based on the weight of the composition, of a C.sub.7 -C.sub.40 oil; and PA1 (b) about 0.1-5.0 wt. %, preferably 0.4-4 wt. %, based on the weight of the composition, of a soap of a tall oil fatty acid.
In the absence of water, the image/non-image distinction is not, however, sufficient to stop printing ink, with its lower surface tension, from wetting both areas. Hence the need for an aqueous fountain solution in lithography to generate a barrier between the ink and non-image area.
The foregoing explanation of lithography is highly simplified since it is based on a static situation. In practice, however, the situation is complicated because ink and aqueous fountain solution are being continuously applied from roller systems to the printing plate.
The aqueous fountain solution is used to maintain the non-image areas of a lithographic printing plate non-receptive to ink. While an offset printing press is running, aqueous fountain solution is continuously applied to the printing plate just before the application of the printing ink, or as a water in ink emulsion. The aqueous fountain solution has an affinity for the non-image, hydrophilic areas of the plate and immediately wets these areas. A complete and uniform film of aqueous fountain solution prevents the subsequent application of ink from covering the plate in a non-image area. The aqueous fountain solution and ink on the plate are then both transferred to the blanket and then to the printing substrate and the process begins again.
Lithographic printing plates are developed to expose a hydrophilic metal surface in the non-image areas, while image areas are left with a hydrophobic surface. There are many aqueous fountain solutions which will wet and coat the exposed metal surface of the non-image area of the plate. Plain water may temporarily perform fairly well, although aqueous solutions of various electrolytes, surfactants and water soluble polymers are generally required for good continuous performance. These additives promote plate wetting and fountain solution uniformity, as well as controlling the interaction of the fountain solution with the ink and the substrate.